1. Field of the Invention
The present invention relates to a light emitting device, and more specifically, to a white light emitting device which is usable for a general illumination and a flash with improved color rendering and is provided with a phosphor having stable chemical characteristics against the external environment such as moisture.
2. Discussion of the Background
A light emitting diode (LED) is a compound semiconductor having a p-n junction structure and refers to a device for emitting light through recombination of minority carriers (electrons or holes). The light emitting diodes have characteristics of low power consumption, long life span, installation in a narrow space, and a strong resistance against vibration. In recent years, white light emitting devices in addition to single color light emitting diodes, e.g. red, blue and green light emitting diodes, have been placed on the market and their demands have been rapidly increased.
A light emitting diode may implement white light by using a phosphor which serves as a wavelength conversion means. That is, the phosphor is applied on a light emitting diode, so that the color of a portion of primary light emitted from the light emitting diode and the color of secondary light whose wavelength has been converted by the phosphor are mixed to implement white color. The white light emitting diode configured as described above has been widely used since it is inexpensive and is very simple in principle and construction.
For example, onto a blue light emitting diode is applied a phosphor that emits yellowish green or yellow light using a portion of the blue light emitted from the blue light emitting diode as an excitation source, so that white light can be obtained by mixing the blue light emitted from the light emitting diode and the yellowish green or yellow light emitted from the phosphor. However, there is a problem in that since such a white light emitting diode employs a single yellow phosphor, spectrum shortage of green and red regions happens. Therefore, color rendering of the above white light emitting diode becomes low.
In order to solve the aforementioned problem, a blue light emitting diode and a phosphors excited by the blue light to emit green and red light are used to manufacture a white light emitting diode. That is, the blue light and the green and red light emitted from the excitation by the blue light may be mixed to implement white light with a high color rendering of 85 or more.
As the red light emitting phosphor, a sulfide based phosphor such as (Ca,Sr)S:Eu and (Zn,Cd)(S,Se):Ag has been typically employed. The sulfide based phosphor may control the wavelength conversion in the range of 600 nm to 660 nm depending on its composition, so that high color rendering may be obtained. However, there is a problem in that the sulfide based phosphor easily reacts with moisture and carbon dioxide in the atmosphere and is then changed into oxide or carbonate, and thus, their chemical characteristics are changed.
Further, H2S gas generated by the reaction with the phosphor and moisture changes the light emitting spectrum of the phosphor to cause a change in color coordinate and a rapid decrease in intensity and corrodes the metal such as Ag or Au formed into an electrode to lower the reliability of the light emitting diode.
A thiogallate based phosphor such as (Ca,Sr,Ba)(Al,Ga,In)2S4:Eu has been typically used as a green light emitting phosphor. The thiogallate based phosphor has excellent excitation efficiency by blue light but the thiogallate based phosphor included in a sulfide group also easily reacts with moisture, and the chemical characteristics of the phosphor are changed.